Those skilled in hydrocarbon recovery operations have long recognized the benefits of quickly and reliably interconnecting a downhole tubular, such as production tubing, with a fixed downhole tool, such as a packer that has been set in the well bore. For various reasons, it may be desirable or essential to set a packer within a well bore, then subsequently connect the previously connected tubular string or another tubular string to the set packer. Typically this interconnection of the production tubing string and the packer is accomplished with an anchor tubing seal assembly, and various models of anchor tubing seal assemblies have been devised for interconnection with specific types of packers. A structural interconnection may thus be made between internal threads on the upper end of a packer and mating external threads on the anchor tubing,, seal assembly suspended from the lower end of a production tubing string. The same anchor tubing seal assembly may be used to structurally interconnect downhole equipment other than packers with various types of tubular goods, such as tail pipes. A subsequent disconnection between the production tubing string and the packer may be accomplished by rotating the tubular string in the direction which will not break apart the threads along the production tubing string.
Anchor tubing seal assemblies have for decades employed a collet that is axially movable relative to the anchor assembly mandrel to interconnect the anchor tubing seal assembly and the upper mandrel of a packer or similar downhole tool fixed within the well bore. The anchor assembly collet fingers may have exterior threads for mated engagement with the interior threads on the upper mandrel of a packer. When the collet fingers are axially in their engaged position, the collet fingers are prevented from moving radially inward relative to the anchor assembly mandrel threads, so that a fixed structural engagement of the collet threads and the packer mandrel threads are obtained. While in the disengaged position, the collet fingers are allowed to move radially relative to the anchor assembly mandrel threads, so that the collet fingers may ratchet down the packer mandrel threads as the anchor assembly mandrel moves axially with respect to the set packer. The collet may be shifted axially to its engaged position by picking up on the production tubing string, so that a tapered surface in the anchor assembly mandrel prevents the collet fingers from moving radially inward, thereby reliably interconnecting the packer and the production tubing string. Production fluid may then be transmitted through the packer and into the production tubing, which is sealingly connected to the packer by a seal assembly in the anchor assembly. To release the production tubing from the packer, the production tubing; and the interconnected collet fingers may be rotated at the surface to unthread the anchor assembly from the packer.
The acceptance of anchor tubing seal assemblies has been limited in part because high axial forces cannot be reliably transmitted through the interconnected anchor tubing seal assembly. If the well operator desires to transmit high axially "pick up" force on the packer, the taper of the acute angle on the engaging collet and the mating anchor assembly mandrel affects the amount of radially outward force placed on the upper threads of the packer mandrel. If these radially inward forces become too high, the tubing mandrel may collapse, resulting in an expensive replacement operation. To lower this collapsing force on the tubing mandrel, the angle of this taper may be increased, e.g., from 30.degree. to 45.degree. . As this angle is increased, however, the bearing surface area between the collet fingers and the anchor assembly mandrel decreases, which decreases bearing strength. Since high stresses must be transmitted through these bearing surfaces, a decrease in the surface area increases the likelihood of permanent bearing surface deformation and thus beating failure.
The disadvantages of the prior art are overcome by the present invention. Improved techniques are hereafter disclosed for reliably interconnecting a tubular member, such as production tubing, to a fixed downhole tool, such as a packer, while enabling high axial forces to be applied to the fixed downhole tool through the tubular member without risking collapse of the tubing mandrel.